With rapid development in various information devices from mobile terminals to large-capacity server, more and more improved performance such as high integration, high speed and low power consumption regarding the memory, logic, and other elements for making up such information devices has been pursued.
Specifically, remarkable progress has been made in semiconductor non-volatile memories, and flash memories as large-capacity file memories have come into wide use as if they can replace hard disks.
Meanwhile, considering expansion of the development into code storage and working memories as well, development of semiconductor non-volatile memories has been underway to replace NOR flash memories, DRAMs, and the like which have been currently generally used. Examples of the semiconductor non-volatile memories include FeRAM, (Ferroelectric Random Access Memory), MRAM (Magnetic Random Access Memory), PCRAM (phase change RAM) and the like. Some of them have already been implemented.
Among these non-volatile memories, MRAMs are capable of high-speed rewriting, and also almost infinite times (more than 1015 times) of rewriting, for they perform data storage by using magnetization direction of a magnetic material. They have already been utilized in the field of industrial automation, aircrafts and the like.
For their high-speed operation and reliability, MRAMs are expected to be developed in code storage and working memories hereafter.
However, MRAMs may still have a problem in achieving low power consumption and large capacity.
This may be a substantial problem coming from the principle of recording in MRAM, that is, a method of reversing magnetization by a current magnetic field produced by a wiring.
One example of the methods to cope with this problem under study is a recording method which does not depend on a current magnetic field (i.e., magnetization reversal). Among such studies, those regarding spin-torque magnetization reversal have been actively carried out (for example, see Patent Documents 1 and 2).
A storage element using the spin-torque magnetization reversal may be configured by MTJ (Magnetic Tunnel Junction), which is the same as in MRAMs.
This utilizes a fact that when spin-polarized electrons passing through a magnetic layer fixed to a certain orientation enter another magnetic free layer (without fixed orientation), the spin-polarized electron exert spin torque on the magnetic free layer. When a current above a certain threshold flows, a free magnetization layer (storage layer) would reverse its direction of magnetization.
Rewriting of 0/1 is made by changing the polarity of a current which is allowed to flow in the storage element.
An absolute value of the current for reversal of the direction of magnetization of the free magnetization layer is typically 1 mA or less in a 0.1 μm-scale storage element. Moreover, as this value of the current decreases proportionally to a volume of the storage element, scaling may be possible.
Furthermore, as it eliminates the need of word lines for producing the write current magnetic field, it may have an advantage that a cell structure can be simple.
Hereinafter, an MRAM using the spin-torque magnetization reversal will be referred to as “STT-MRAM (Spin Transfer Torque-Magnetic Random Access Memory)”.
There is a great expectation in STT-MRAMs as non-volatile memories which are capable of realizing low power consumption and large capacity while keeping advantages of MRAMs where high speed and almost infinite times of rewriting is available.    Patent Document 1: Japanese Patent Application Laid-open No. 2003-17782    Patent Document 2: U.S. Pat. No. 5,695,864